1. Field of the Invention
This invention relates generally to an amplifier circuit for use in an audio system, and in particular to an amplifier circuit with substantially zero distortion.
2. Prior Art
There has been proposed a low distortion amplifier in which a feedback path including a distortion detection amplifier is coupled to a main amplifier to reduce distortion generated therein. There is shown in FIG. 1 such a conventional low distortion amplifier 1. This amplifier 1 essentially comprises an addition circuit 2, a main amplifier 3, an impedance circuit 4 and a distortion detection circuit 5. An input signal Ei applied to an input terminal 6 is fed via the addition circuit 2 to an input terminal of the main amplifier 3 and an output signal Eo of this main amplifier 3 is derived from an output terminal 7. This output signal Eo is, on the other hand, supplied via the impedance circuit 4 to an inverting input terminal of the distortion detection amplifier 5 whose non-inverting input terminal is supplied with the input signal Ei. And an output signal of this distortion detection amplifier 5 is applied to the addition circuit 2.
With this construction, the following equation indicative of the relation between the input signal Ei and output signal Eo is established: EQU (Ei+(Ei-Eo.multidot..beta.).multidot.B)A=Eo (1)
where A is voltage gain of the main amplifier 3, B voltage gain of the distortion detection amplifier 5, and .beta. a feedback factor of the impedance circuit 4. This equation (1) gives a transfer function Gv of this amplifier 1 as follows: ##EQU1## The above equation (2) shows that the transfer function Gv becomes ( 1/.beta.) when the voltage gain B is infinite. This means that even if the main amplifier 3 generates an amount of distortion, distortion contained in the output signal Eo can be reduced to a negligible amount by setting the gain B of the distortion detection amplifier 5 to a very large value. With this amplifier 1, also, the feedback signal introduced from the output thereof into the impedance circuit 4 is first amplified by the distortion detection amplifier 5 and then again amplified by the main amplifier 3 before returning to the same output. The stability of this amplifier 1 can therefore be affected not only by the frequency response of the main amplifier 3 but also by that of the distortion detection amplifier 5. In general, an amplifier having a good phase response even at high frequencies is of a low-power type and a high-power-type amplifier having such a good phase response has not yet been available. Incidentally, the distortion detection amplifier 5 shown in FIG. 1 need not perform a power-amplification and may therefore be of a low-power type that can have a good phase response. On the contrary, the main amplifier 3 must perform a power-amplification and must therefore be of a high-power type, however such a high-power-type amplifier is generally poor in phase response. For this reason, it has been quite difficult to improve at high frequencies the stability of this kind of conventional low-distortion amplifier. And as a result, such a conventional low-distortion amplifier is liable to oscillate at high frequencies.